Billionth Second Days occur on the days that you are a whole billion seconds old.

So, on your first Billionth Second Day you have been alive for 1,000,000,000 seconds.

Strictly speaking this is a 'secundiversary' rather than a 'dieversary',
but in order to commemorate it as a secundiversary you would need to know the precise second that you were born,
and you would only be able to celebrate it for one second, rather than the whole day.

For this reason we classify it as a dieversary and recommend that you spend the entire day celebrating it.